Support task dependencies and blocking relationships

[Valentin-v-Todorov]

Allow tasks to depend on other tasks being completed first.

• Add --depends-on flag
• Show blocked tasks differently in ls output
• Prevent completing tasks with incomplete dependencies
• Add 'tix deps' command to visualize dependency graph

[vj031206]

@Valentin-v-Todorov I am willing to work on this issue under hacktoberfest? Could i be assigned this issue?

[Valentin-v-Todorov]

@vj031206 Thanks for collaborating !!